Acids and Bases - The pH Scale and Strong Acids (A-Level Chemistry)

The pH Scale and Strong Acids

The pH Scale

Measuring the pH of Strong Acids

The strength of acids can be measured by measuring the concentration of H+ ions.

However, the concentration of hydrogen ions can be very small in some solutions. It is therefore more appropriate to use a logarithmic scale, known as the pH scale.

The pH Scale

The formula for calculating pH is:

Where [H+] is the concentration of H+ ions in mol dm-Ā³.

The pH scale ranges from 0 (very acidic) to 14 (very basic). A pH of 7 is considered as neutral. Water and solutions of salts have neutral pHs.

Each unit of the pH scale represents a 10 fold change in the hydrogen ion concentration. For example pH 2 has ten times more hydrogen ions compared to pH 3.

pH of Equimolar Solutions

Equimolar solutions are solutions containing the same number of moles of different substances.

In order to know whether a substance is an acid, a base or a salt simply measure and compare the pH of different equimolar solutions (i.e.: calculate the pH of 1 mol dm-3 solutions of each substance)

Calculations involving pH

Using pH Formula with Strong Acids

For strong monoprotic acids like hydrochloric acid (HCl) or nitric acid (HNO3), the number of moles of acid = the number of moles of hydrogen ions.

For strong diprotic acids, for example sulfuric acid, two moles of hydrogen ions are released for each mole of acid. The number of moles of acid x 2 = the number of moles of hydrogen ions.

Calculating pH using H+ concentration

The example below shows how to calculate pH given the hydrogen ion concentration of a strong acid.

Worked Example: Calculate the pH of a solution containing 0.01 mol of sulfuric acid in 50 cmĀ³. Give your answer to 2 significant figures.

Answer:

1. Work out the concentration of the solution in mol dm-Ā³. Remember to convert all values to the same units.

There are 0.01 mol in 50 cmĀ³, so there are 0.01/50 x 1000 in 1 dmĀ³ = 0.2 mol dm-Ā³

2. Work out the concentration of hydrogen ions. Remember to differentiate between monoprotic and polyprotic acids. Sulfuric acid (H2SO4) is a diprotic acid: for each mole of acid, there are two moles of hydrogen ions.

Therefore, the concentration of hydrogen ions is 2 x 0.2 = 0.4 mol dm-Ā³.

3. Work out the pH of the solution using the concentration of hydrogen ions. Apply the formula above.

pH = -log10(0.40) pH = 0.40

Calculating H+ concentration using pH

The next example shows how to calculate the concentration of hydrogen ions given the pH.

Worked Example: Calculate the concentration of hydrogen ions in a solution of hydrochloric acid with a pH of 1.5

Answer:

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